7 expressions and assignment statements

ISBN 0-321-49362-1

Chapter 7

Expressions and Assignment Statements

Copyright © 2007 Addison-Wesley. All rights reserved. 1-2

Chapter 7 Topics

• Introduction• Arithmetic Expressions• Overloaded Operators• Type Conversions• Relational and Boolean Expressions• Short-Circuit Evaluation• Assignment Statements• Mixed-Mode Assignment


Introduction

• Expressions are the fundamental means of specifying computations in a programming language

• To understand expression evaluation, need to be familiar with the orders of operator and operand evaluation

• Essence of imperative languages is dominant role of assignment statements


Arithmetic Expressions

• Arithmetic evaluation was one of the motivations for the development of the first programming languages

• Arithmetic expressions consist of operators, operands, parentheses, and function calls


Arithmetic Expressions: Design Issues

• Design issues for arithmetic expressions– Operator precedence rules?– Operator associativity rules?– Order of operand evaluation?– Operand evaluation side effects?– Operator overloading?– Type mixing in expressions?


Arithmetic Expressions: Operators

• A unary operator has one operand• A binary operator has two operands• A ternary operator has three operands


Arithmetic Expressions: Operator Precedence Rules

• The operator precedence rules for expression evaluation define the order in which “adjacent” operators of different precedence levels are evaluated

• Typical precedence levels– parentheses– unary operators– ** (if the language supports it)– *, /– +, -


Arithmetic Expressions: Operator Associativity Rule• The operator associativity rules for expression

evaluation define the order in which adjacent operators with the same precedence level are evaluated

• Typical associativity rules– Left to right, except **, which is right to left– Sometimes unary operators associate right to left (e.g.,

in FORTRAN)• APL is different; all operators have equal

precedence and all operators associate right to left

• Precedence and associativity rules can be overriden with parentheses


Ruby Expressions

• All arithmetic, relational, and assignment operators, as well as array indexing, shifts, and bit-wise logic operators, are implemented as methods

- One result of this is that these operators can all

be overriden by application programs


Arithmetic Expressions: Conditional Expressions• Conditional Expressions

– C-based languages (e.g., C, C++)– An example:

average = (count == 0)? 0 : sum / count

– Evaluates as if written likeif (count == 0)

average = 0else

average = sum /count


Arithmetic Expressions: Operand Evaluation Order• Operand evaluation order

1. Variables: fetch the value from memory2. Constants: sometimes a fetch from memory;

sometimes the constant is in the machine language instruction

3. Parenthesized expressions: evaluate all operands and operators first

4. The most interesting case is when an operand is a function call


Arithmetic Expressions: Potentials for Side Effects• Functional side effects: when a function changes

a two-way parameter or a non-local variable• Problem with functional side effects:

– When a function referenced in an expression alters another operand of the expression; e.g., for a parameter change:

a = 10; /* assume that fun changes its parameter */

b = a + fun(a);


Functional Side Effects

• Two possible solutions to the problem1. Write the language definition to disallow

functional side effects• No two-way parameters in functions• Advantage: it works!• Disadvantage: inflexibility of one-way

parameters Write the language definition to demand that operand evaluation order be fixed

• Disadvantage: limits some compiler optimizations

• Java requires that operands appear to be evaluated in left-to-right order


Overloaded Operators

• Use of an operator for more than one purpose is called operator overloading

• Some are common (e.g., + for int and float)

• Some are potential trouble (e.g., * in C and C++)– Loss of compiler error detection (omission of an

operand should be a detectable error)– Some loss of readability– Can be avoided by introduction of new symbols

(e.g., Pascal’s div for integer division)


Overloaded Operators (continued)

• C++, Ada, Fortran 95, and C# allow user-defined overloaded operators

• Potential problems: – Users can define nonsense operations– Readability may suffer, even when the

operators make sense


Type Conversions

• A narrowing conversion is one that converts an object to a type that cannot include all of the values of the original type e.g., float to int

• A widening conversion is one in which an object is converted to a type that can include at least approximations to all of the values of the original type e.g., int to float


Type Conversions: Mixed Mode

• A mixed-mode expression is one that has operands of different types

• A coercion is an implicit type conversion• Disadvantage of coercions:

– They decrease in the type error detection ability of the compiler

• In most languages, all numeric types are coerced in expressions, using widening conversions

• In Ada, there are virtually no coercions in expressions


Explicit Type Conversions

• Called casting in C-based languages• Examples

– C: (int)angle– Ada: Float (Sum)

Note that Ada’s syntax is similar to that of function calls


Type Conversions: Errors in Expressions• Causes

– Inherent limitations of arithmetic e.g., division by zero

– Limitations of computer arithmetic e.g. overflow

• Often ignored by the run-time system


Relational and Boolean Expressions• Relational Expressions

– Use relational operators and operands of various types

– Evaluate to some Boolean representation– Operator symbols used vary somewhat among

languages (!=, /=, .NE., <>, #)• JavaScript and PHP have two additional

relational operator, === and !== - Similar to their cousins, == and !=, except that

they do not coerce their operands


Relational and Boolean Expressions• Boolean Expressions

– Operands are Boolean and the result is Boolean

– Example operators

FORTRAN 77 FORTRAN 90 C Ada .AND. and && and .OR. or || or .NOT. not ! not xor


Relational and Boolean Expressions: No Boolean Type in C• C89 has no Boolean type--it uses int type

with 0 for false and nonzero for true• One odd characteristic of C’s expressions:

a < b < c is a legal expression, but the result is not what you might expect:– Left operator is evaluated, producing 0 or 1– The evaluation result is then compared with

the third operand (i.e., c)


Short Circuit Evaluation

• An expression in which the result is determined without evaluating all of the operands and/or operators

• Example: (13*a) * (b/13–1)If a is zero, there is no need to evaluate (b/13-1)

• Problem with non-short-circuit evaluationindex = 1;while (index <= length) && (LIST[index] != value) index++;– When index=length, LIST [index] will cause an

indexing problem (assuming LIST has length -1 elements)


Short Circuit Evaluation (continued)• C, C++, and Java: use short-circuit evaluation for

the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |)

• Ada: programmer can specify either (short-circuit is specified with and then and or else)

• Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3)


Assignment Statements

• The general syntax<target_var> <assign_operator> <expression>

• The assignment operator= FORTRAN, BASIC, the C-based languages:= ALGOLs, Pascal, Ada

• = can be bad when it is overloaded for the relational operator for equality (that’s why the C-based languages use == as the relational operator)


Assignment Statements: Conditional Targets• Conditional targets (Perl)($flag ? $total : $subtotal) = 0

Which is equivalent to

if ($flag){$total = 0

} else {$subtotal = 0

}


Assignment Statements: Compound Operators• A shorthand method of specifying a

commonly needed form of assignment• Introduced in ALGOL; adopted by C• Example

a = a + b

is written as

a += b


Assignment Statements: Unary Assignment Operators

• Unary assignment operators in C-based languages combine increment and decrement operations with assignment

• Examplessum = ++count (count incremented, added to sum)

sum = count++ (count incremented, added to sum)

count++ (count incremented)-count++ (count incremented then negated)


Assignment as an Expression• In C, C++, and Java, the assignment

statement produces a result and can be used as operands

• An example: while ((ch = getchar())!= EOF){…}

ch = getchar() is carried out; the result (assigned to ch) is used as a conditional value for the while statement


List Assignments

• Perl and Ruby support list assignments e.g., ($first, $second, $third) = (20, 30, 40);


Mixed-Mode Assignment• Assignment statements can also be mixed-

mode, for exampleint a, b;float c;c = a / b;

• In Fortran, C, and C++, any numeric type value can be assigned to any numeric type variable

• In Java, only widening assignment coercions are done

• In Ada, there is no assignment coercion


Summary

• Expressions• Operator precedence and associativity• Operator overloading• Mixed-type expressions• Various forms of assignment

7 expressions and assignment statements

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